This invention relates to the writing of sector servo patterns on disk drives, and in particular, to an improved method for writing such patterns using an inertial sensor to position the head of the drive during writing.
This disclosure relates to the general problem of writing sector servo patterns on hard disk drives. Today, nearly all disk drives rely on sector servo patterns for accurately controlling the position of the head during both reading and writing. The head flies over a specially written servo burst that provides it with an extremely accurate indication of the head""s position relative to the track center. Assuming that the head is close to the center of the track, it can then either read or write the following data sector. If the head is off track, corrective feedback is applied by the servo loop using the voice-coil actuator. As long as the frequency with which the head flies over sector servo bursts is high enough, the head can be kept at the center of the track to within a small fraction of the track width.
Typically these servo bursts are quite complicated patterns that are written by the same write head normally used in the disk drive to read and write data. One typical pattern is to write the inner half of the track with a tone at one frequency and the outer half of the track with a tone at a second frequency. Because the head always writes a full track width, this pattern must be written from the inside to the outside (or vice versa) with the head stepping over by just half a track each time the frequency is switched. More advanced disk drives sometimes use more complicated servo marks. For example, patterns that make use of three or four distinct frequencies can be used instead of using just two frequencies.
Generally, to write servo marks, the head is moved to a precise radius and then held at that radius while a sequence of patterns is written for each of the sector servo marks in that track, as the disk rotates. After a full rotation, the head must be moved inward or outward by a precise amount and then held at that exact radius while sector servo marks for the next track are written.
Currently, nearly all hard disk drives have the sector servo pattern written onto the disk using its own heads, using a laser interferometer to sense the position of the head. During the servo writing process, which is typically done shortly after the drive is manufactured, the disk is typically spun at a fraction of its normal rotational speed (typically about half of the normal operating speed) to minimize bearing vibrations and air turbulence vibrations. To isolate the disk drive from external vibrations the servo writing process is typically carried out in a vibration damped environment.
The fact that the track density of disk drives is increasing rapidly presents a problem with the current state of the art practice. A disk drive that has 40,000 tracks and operates at a rotational speed of 12,000 RPM, with a simple dual frequency servo pattern, would require at least 400 seconds, or over 6 minutes, to complete the writing of the servo pattern. As track densities increase, this time scales up linearly, which could make servo writing the most expensive step in manufacturing a disk drive.
In the early days of disk drives, positioning of the read/write head was done using stepper motors. The position of the arm was precisely known by the number and direction of step commands that had been given to the stepper motor. Because of this feature, early disk drives were capable of self servo writing, even in the field (this was called the xe2x80x9cformatxe2x80x9d command). As voice-coil motors replaced the stepper motors, it became necessary to have some kind of additional track position reference. Although many different ways have been developed to provide this position reference, the use of a laser interferometer based sensor to control the head position and the one-time writing of dedicated servo marks has emerged as the industry standard approach.
Because of the high cost of time on today""s laser interferometer based servo writing machines, there have long been efforts to create a xe2x80x9cself servo writingxe2x80x9d mechanism that allows the disk drive to write the sector servo patterns itself, without the use of a stepper motor to set the position of the head.
There are several problems with current methods of doing self-servo writing without the use of a stepper motor. One such problem involves the positioning of the write head as the marks are written on the disk. After a single mark is written, the write head must be held motionless until the spot on the disk for the next mark rotates under the write head. Because there is no inertial frame of reference, the write head is xe2x80x9copen loopxe2x80x9d for this period of time. As a result, a certain percentage error in the positioning of the write head occurs, usually not more than one percent (1%) per track. However, the positioning error in each track is cumulative over the entire surface of the disk, and, as a result, large errors may be induced near the outside diameter of the disk, with the result that the tracks near the outside diameter of the disk may be untrackable by the read head of the drive.
As an example, if the first track of a disk drive is written near the inside diameter of the disk using the inside crash stop as a reference, the sector servo marks in the first track can be written in a fairly accurate manner. However, as the write head moves outward to write the sector servo marks on the next track, the only reference the head has as to where to write the second track of sector servo marks is the previous track. It is not unlikely that positioning errors of up to one (1%) percent per track could occur as the write head moves outwardly from track to track. To further complicate the process, the write head moves in an xe2x80x9copen-loopxe2x80x9d fashion from servo mark to servo mark as it writes the servo marks on the current track. Certain forces acting upon the write head, such as windage caused by the spinning of the disk and forces exerted by, for example, the cable attached to the write head are likely to cause positioning errors even between servo marks in the same track.
It is therefore desirable to provide a method of writing sector server marks that eliminates or minimizes positioning errors.
The present invention consists essentially of providing an inertial sensor to sense any acceleration forces applied to the head. Preferably, the inertial sensor is positioned on the arm to which the head is attached.
The head preferably holds a constant radial position as servo marks in the same track are written. Heads on current state-of-the-art drives are unable to do this for more than the distance circumferentially between one or two sector servo marks even when drive current to the actuator is turned off because of residual forces due to air moving by the head (windage forces) and drag from the cable attaching the arm electronics to the rest of the drive. That is, the head can be assumed to remain at a fixed radius when the actuator drive current is turned off only for a period of time corresponding to the time between one or two sector servo bursts. The inertial sensor is able to detect inward or outward motions of the head and, using a force feedback mechanism, is able to eliminate or minimize there radial movements of the head as servo marks in the same track are written for periods much longer than the time between sector servo bursts.
The inertial sensor is also used when moving the head to a different track. A calibration process, described in detail later, can be used to calculate a combination of accelerating and decelerating forces that can be applied to the head to move it from its current position to the correct position for the writing of the next track. When it is time to move the head to the next track, an accelerating force is applied to the head for a predetermined period of time, guided by the inertial sensor. Then a decelerating force is applied for a precise period of time to stop the movement of the head. The result is that the head will be resting over the portion of the disk on which the servo marks for the next track are to be written. When the new position is reached, the previously described force feedback mechanism is used to keep the head from deviating from the radial position of the new track. Alternatively, the internal force actuators within the inertial sensor can be used to move the proof mass that is part of the sensor over by a precise amount as determined by the position sensing electronics within the inertial sensor. Then, the overall servo feedback loop can be used to recenter the arm, and hence the sensor""s package, around the proof mass of the inertial sensor.